ppc64/crtools.c: fix compilation with clang

For gcc, having "inline" is enough to suppress the "unused function"
warning. For clang, we have to do better than that.

See also: commit 4241078 ("zdtm: compile fix for clang") and
commit 3ea2fd78 ("crtools.c: fix compilation by clang").

travis-ci: success for PPC+clang compile fixes
Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Acked-by: Laurent Dufour <ldufour@linux.vnet.ibm.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>